In recent years, in association with improvement in image processing technology as well as in image forming technology it has become possible to form an image so precisely that a copied bill copied with a color copying machine can not easily be differentiated from an actual bill. For this reason there has been developed an apparatus for determining a specific document such as bills or marketable securities inhibited from be copied and for inhibiting an illegal operation for copying when it is determined that the document is a specific document, and also there has been proposed an image forming apparatus such as a copying machine with a function for determining a specific document incorporated therein.
As examples of the specific document as described above, there are bills with metallic fiber included therein to indicate that the bills are specific documents. To differentiate the specific documents from ordinary documents as described above, it is required only to detect metallic fiber included in a document. For this purpose, as an image forming apparatus having a function to determine a specific document with metallic fiber included therein, there has been proposed an apparatus comprising a microwave sensor for irradiating a microwave to a document, detecting a transmitted wave or a reflected wave of the irradiated microwave, and outputting a detection signal, and a signal determining section for receiving the detection signal outputted from the microwave sensor and making determination as to whether metallic fiber is included in the document or not.
The microwave sensor in the image forming apparatus, provided generally in an image reading unit having a CCD or the like for photographing, reciprocates for scanning together with the image reading unit over a document placed on the document base, irradiates a microwave to the document for detecting metallic fiber and detects a reflected wave in the forward direction for scanning. Then the signal determining section receives the detection signal for the reflected wave outputted from the microwave sensor, compares a level of the received detection signal to a preset threshold value, and determines that metallic fiber is included in the document when the level of the detection signal exceeds the threshold value, and also determines that metallic fiber is not included in the document when the level of the detection signal does not exceed the threshold value. As a result, it is determined that the document is a specific document such as bills or marketable securities with metallic fiber included therein when it is determined that metallic fiber is included in the document.
However, in the conventional technology, for example, when a metallic member is included in a pressure plate, when an attached condition of the pressure plate is changed, or when a use environment of the apparatus is changed, a level of a detection signal outputted from the microwave sensor may change, which changes accuracy in detection of metallic fiber included in a document.
Also, in the conventional technology, any interference elements such as a noise due to an influence of vibrations of the image reading unit and changes in temperature or the like may sometimes be included in the detection signal outputted from the microwave sensor, so that it is not always possible to make determination of a specific document with high accuracy.
In the conventional technology, during the operation for determining as to whether a document is a specific document or not, a user may unintentionally open the pressure plate. In this case, a microwave may directly be irradiated to the user, and for this reason, it is necessary to design the apparatus so as not to directly irradiate a microwave to a human body.
In the conventional technology, when the microwave sensor is not operating normally, metallic fiber included in a document can not accurately be detected, so that accuracy in determination of a specific document may lower.
In the image forming apparatus based on the conventional technology, when any abnormality occurs in a specific document determining apparatus due to a breakdown thereof so that determination is not made as to whether the document is a specific document or not, an operation for copying ordinary documents can not be performed until the specific document determining apparatus is returned to normal operation because action for preventing illegal copying of a specific document is taken by inhibiting whole operations for copying.
Especially, even though illegal copying of bills or marketable securities generally requires color copying, with the conventional technology, copying in a monochrome copy mode used with high frequency is also inhibited in a copying machine having both a color copy mode and a monochrome copy mode, so that it is not possible to make effective use of a copying machine having a color copy mode as well as a monochrome copy mode.